This invention relates to stereo demodulator circuitry for use in a pilot tone, FM stereo broadcasting system.
In conventional stereo demodulator circuits, it is known that the second harmonic component of the subcarrier contained in the output signal and the SCA (Subsidiary Communication Authorization) signal, which is ordinarily used, mutually interfere, causing so-called SCA beat interference. In other words, the frequency of the subcarrier is normally 38 KHz. Thus, the 76 KHz second harmonic and the SCA signal frequency band, 60-74 KHz (subcarrier is 67 KHz), tend to develop undesired beat signals in the audio frequency zone and mix with the desired reproduced sound causing strong interference.
In order to eliminate this SCA beat interference, an SCA signal elimination filter is conventionally employed in the stage previous to the stereo demodulator circuit to remove the SCA signal from the stereo composite signal. However, the stereo composite signal is subjected to a phase deviation because of this filter. Consequently, a decrease in the separation between the left and right signals and a deterioration of frequency characteristics are unavoidable. It thus is necessary to insert a shift circuit on the output side of the filter or install a cross-talk blanking circuit after the stereo demodulator circuit to compensate for the decreased separation, and to correct the frequency characteristic with a tone control circuit, etc. However, in all cases, the adjustment is complicated and a superior effect cannot be expected.